Adaptable solar airframe with a flexible photovoltaic system

ABSTRACT

Methods and apparatus for an adaptable solar airframe are provided herein. In some embodiments, an adaptable solar airframe includes an expandable body having an aerodynamic cross-section that reduces parasitic air drag at any given thickness of the body, further being able to change its shape in flight in response to changes in the relative position of the sun; and a flexible solar PV system attached to the surface of the expandable body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/747,626, filed Jan. 23, 2013. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

FIELD

Embodiments of the present invention generally relate to the improveduse of solar energy on aircraft.

BACKGROUND

Unmanned Aerial Vehicles (UAVs) are unpiloted aircraft that are eithercontrolled remotely or are autonomously flown based on pre-programmedflight plans. UAVs are also commonly categorized based on their designand performance specifications that span the range from miniature lowaltitude to large High Altitude Long Endurance (HALE) vehicles. HALEUAVs could provide improved service over existing systems in a largenumber of civil applications, ranging from border patrol and coastalsurveillance, monitoring of natural disasters, meteorology andcartography to highly flexible telecommunication relay stations. Forexample, platforms capable to remain airborne for weeks to months ataltitudes of about 10-25 km provide advantages over satellite systems interms of reduced costs, increased flexibility and higher precision.

The UAV technology is taking an increasingly important place in oursociety for civilian and military applications. The required enduranceis in the range of a few hours in the case of law enforcement, bordersurveillance, forest fire fighting or power line inspection. However,other applications at high altitudes, such as communication platform formobile devices, weather research and forecast, environmental monitoring,would require remaining airborne for days, weeks, months or even years.It is possible to reach these goals using electric solar poweredplatforms. Photovoltaic (PV) cells and modules may be used to collectthe solar energy during the day, a part of which may be used directlyfor maintaining flight and onboard operations with the remainder beingstored for the night time.

The use of sunlight as a source of energy for aircraft has manycompelling advantages. Solar energy entails zero marginal cost, weight,and emissions per hour of flight. Sunlight provides a maximum of about1000 W/m² at sea level, but reaches a more abundant 1400 W/m² at highaltitudes unobstructed by cloud cover. With advances in efficient andlightweight materials for collector, storage, and wing structures, solaraircraft can aspire to sustain flight at high altitudes for days, weeks,even years.

One approach to building a solar airplane is to cover the upper wingsurfaces of a plane with photovoltaic cells. This configuration worksbest when the sun is directly overhead, but it suffers a loss of powerproportional to cosine of the angle between the normal of the wingsurface and the sun direction. In 1998 the DLR Institute of FlightSystems built the “Solitair” prototype with solar panels that tilt alonga single axis to orient towards the sun. This approach works best whenthe plane can fly in the direction perpendicular to the solar azimuth,but otherwise also suffers a cosine power loss. Such panels also createturbulence, aerodynamic instability and drag. In 1999-2003,AeroVironment and NASA developed the “Helios” UAV prototype. “Helios”wing is segmented into several solar-cell-covered sections connected byhinged joints. The joints allow tilting some of the sections towards thesun, but do not significantly compensate for the cosine power reduction.

A different approach to a “Solar Thermal Aircraft” is disclosed in U.S.Pat. No. 7,270,295 of C. L. Bennett. As shown in FIG. 7 of the abovereferenced patent, the solar collector is a reflective parabolic trough(110) mounted to rotate freely around its focal axis in an opticallytransparent section of the aircraft body. A solar tracker aligns thereflective trough with the sun, to concentrate sunlight onto a heat pipe(120) along the focal axis, thereby heating a fluid which transferssolar energy to a heat engine (140) that propels the aircraft.

In contrast to other solar aircraft that propose solar collectors withinthe airframe of a plane, Bennett's aircraft body and wing pod designsare aerodynamically inefficient. Bennett proposes the opticallytransparent portion of the fuselage skin to be a strongultraviolet-resistant polymer film, such as DuPont TEDLAR®, which hasexcellent transparency, tensile strength, and low weight. However thefixed drag-to-lift characteristics of Bennett's fuselage design willincrease the propulsion power needed to remain on station, even at nightwhen the solar collectors are idle.

The entire prior art solar aircraft referenced above suffer a cosine lawreduction of power when the sun's direction is not parallel to thenormal of the solar panels. The Bennett trough, “Solitair” panels, andother tilting tail wing designs all provide single-tilt compensationonly for the banking or roll angle of sun relative to the ideal overheadposition. For example, none of these designs can harvest solar powerwhen the aircraft is flying directly towards or away from the sun.Tilting panels and wings should not provide aerodynamic lift, becauseresulting forces would compromise aerodynamic stability and may evendamage the aircraft. Thus, such tilting collectors are essentially puredrag elements, and like the Bennett fuselage will increase propulsionpower needed to remain on station, including at night time when thesolar panels are idle.

SUMMARY

Embodiments of the present invention generally relate to the improveduse of solar energy on aircraft. In particular, the invention relates toan aircraft employing a solar power system integrated into an airframethat adapts the system's orientation and shape to optimize collection ofsunlight under varying conditions. The airframe provides streamlining toimprove aerodynamic drag and stability, and to relieve wind load andstructural weight of the collectors. Under varying sunlight conditions,the airframe and collector are modifiable in flight into a multiplicityof streamlined configurations, to reduce drag and remain on station overlarger ranges of time, weather, altitude, and latitude.

At least some embodiments of the present invention may advantageouslyreduce aerodynamic drag, particularly under low-light or nighttimeconditions, by reducing the size of solar collectors and an airframecontaining them. The reduction in size can be accomplished byinterleaving or folding segmented rigid elements, or by deflating,rolling, or folding flexible elements in both the collector andairframe.

At least some embodiments of the present invention may advantageouslyimprove solar collection efficiency by compensating both for the rolland pitch angles in order to provide normal incidence for sunlight undermost flight conditions. The roll angle compensation can be accomplished,for example, by banking the solar collectors with an integrated airframearound the axis of flight. The pitch angle compensation can beaccomplished, for example, by increasing the cross-section of theadaptable airframe to provide a greater area of capture of sunlightalong the flight direction. At each pitch angle, the aerofoil shapeadapts to minimize drag for the particular capture area under givenflight conditions.

In some embodiments of the present invention, some airframe surfaces aremade of highly transparent flexible films, such as fluoropolymers, whichadapt as above to varying conditions—but in this environment thecollectors are internal to the aerofoil, and capable of shrinking to acompact size under low-light conditions, or expanding to a larger sizeand to be steered during high illumination/variable incident angleconditions.

In some embodiments of the present invention, the solar collectors aredeposited directly on flexible films that make up a variable portion ofthe aerofoil surfaces. These flexible films adapt by deflating, rollingor folding to reduce aerodynamic drag under low-light conditions, or toprovide near optimal tradeoff with capture area during high illuminationand variable incident angle sunlight conditions.

In some embodiments, the collectors are steered to remain nearlyorthogonal to the incoming sunlight, thereby being capable of higherconversion efficiency by means of reflective or diffraction gratingwave-length division multi-junction photovoltaics.

In some embodiments, an adaptable solar airframe includes a solar PVsystem having at least one solar tracking system and being able tofollow the sun position in order to increase sunlight collection andpower output; and an expandable body having an aerodynamic cross-sectionthat minimizes parasitic air drag at any given thickness of the body,further being at least partially transparent to sunlight, furtherenclosing the solar PV system, and further being able to change itsshape in response to changes in the position of the solar PV system.

In some embodiments, an adaptable solar airframe includes an expandablebody having an aerodynamic cross-section that reduces parasitic air dragat any given thickness of the body, further being able to change itsshape in flight in response to changes in the relative position of thesun; and a flexible solar PV system attached to the surface of theexpandable body.

In some embodiments, an adaptable solar airframe includes an expandablebody having an aerodynamic cross-section that minimizes parasitic airdrag at any given thickness of the body, further being able to changeits shape in response to changes in the relative position of the sun;and a flexible solar PV system attached to the surface of the expandablebody.

In some embodiments, an adaptable solar airframe includes an expandablebody having an aerodynamic cross-section that reduces parasitic air dragat any given thickness of the body, further being able to change itsshape in flight in response to changes in the relative position of thesun; and an integrated solar power system comprising thin-film PV solarcells and modules.

Other embodiments and variations of the invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention, briefly summarized above anddiscussed in greater detail below, can be understood by reference to theillustrative embodiments of the invention depicted in the appendeddrawings. It is to be noted, however, that the appended drawingsillustrate only some embodiments of this invention and are therefore notto be considered limiting of its scope, for the invention may possessother equally effective embodiments.

FIG. 1 depicts a schematic view of the main parts of a typical airplane.

FIG. 2 depicts an adaptable solar airframe comprising an expandableairframe and an integrated solar power system in accordance with someembodiments of the present invention.

FIG. 3 depicts an adaptable solar airframe comprising an expandableairframe and an integrated solar power system in accordance with someembodiments of the present invention.

FIG. 4 depicts an expandable airframe in accordance with someembodiments of the present invention.

FIG. 5 depicts an expandable airframe in accordance with someembodiments of the present invention.

FIG. 6 depicts an airframe comprised of three telescoping sections thatmay be folded or collapsed into a more compact form in accordance withsome embodiments of the present invention.

FIG. 7 depicts the airframe of FIG. 6 folded or collapsed into a morecompact form.

FIG. 8 depicts a solar power system in accordance with some embodimentsof the present invention.

FIG. 9 depicts a solar power system in accordance with some embodimentsof the present invention.

FIG. 10 depicts a solar power system in accordance with some embodimentsof the present invention.

FIG. 11 depicts an adaptable solar airframe in accordance with someembodiments of the present invention.

FIG. 12 depicts an adaptable solar airframe in accordance with someembodiments of the present invention.

FIG. 13 depicts an adaptable solar airframe in accordance with someembodiments of the present invention.

FIG. 14 depicts a solar power system in accordance with some embodimentsof the present invention.

FIG. 15 depicts the head-on view of a plane with an adaptive solarairframe in accordance with some embodiments of the present invention.

FIG. 16 depicts the side view of a plane with an adaptive solar airframein accordance with some embodiments of the present invention.

FIGS. 17A-C (collectively referred to as FIG. 17) respectively depictconfigurations of an adaptive airframe that comprises an expandableairframe and a solar power system having a controllable shape tocompensate for different pitch angle of the sun in accordance with someembodiments of the present invention.

FIG. 18 depicts an adaptable solar airframe made in the form of aninflatable, flexible pod covered with PV solar cells in accordance withsome embodiments of the present invention.

FIG. 19 depicts schematically an expandable airframe comprising anexternal shell or a canopy held in place by expandable struts inaccordance with some embodiments of the present invention.

FIG. 20 depicts schematically an expandable airframe comprising anexternal shell or a canopy held in place by rigid rotating struts inaccordance with some embodiments of the present invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The figures are not drawn to scale and may be simplifiedfor clarity. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

In the present invention, numerous specific details are set forth inorder to provide a thorough understanding of exemplary embodiments orother examples described herein. However, it will be understood thatthese embodiments and examples may be practiced without the specificdetails. In other instances, well-known methods, procedures, componentsand circuits have not been described in detail, so as not to obscure thefollowing description. Further, the embodiments disclosed are forexemplary purposes only and other embodiments may be employed in lieuof, or in combination with, the embodiments disclosed.

FIG. 1 shows schematically the main parts of a typical airplane 100,which include a fuselage 110, a main wing 120 and a tail section 130(the propulsion system is not shown for simplicity). In addition, theremay be other parts that are rigidly or otherwise attached to each otherand the rest of the airplane or towed using a flexible line or a cord.These airplane parts may be streamlined and aerodynamically shaped toreduce aerodynamic resistance or drag.

In accordance with embodiments of this invention, an aerodynamic body isprovided that is optimal as a part of a solar-powered aircraft and itsairframe. FIGS. 2 and 3 schematically show examples of such aerodynamicbodies, which are further referred to as airframes. FIG. 2 shows anadaptable solar airframe 200 comprising an expandable airframe 210 andan integrated solar power system 220. The solar power system 220 is allor in part located inside the volume of the airframe 210. The airframe210 is at least in part transparent to allow sunlight to pass throughits surface towards the solar power system 220, where it is converted toelectricity. Some airframe surfaces may be made from highly transparentflexible films, such as fluoropolymers, with which the airframes canadapt their shape to varying conditions. The solar power system 220 isinternal to the airframe 210 and capable of folding (or otherwiseshrinking) to a compact size under low-light conditions or unfolding (orotherwise expanding) to a larger size and being steered during daytimeunder variable incident light conditions. FIG. 3 also shows an adaptablesolar airframe 300 comprising an expandable airframe 310 and anintegrated solar power system 320. In this case, the solar power system320 is at least in part located on the surface of the airframe 310. Thesolar power system 320 may comprise for example thin-film PV solar cellsand modules that are either attached to the surface of the airframe 310or imbedded into its skin.

In accordance with embodiments of this invention, an expandable airframe400 shown in FIG. 4 may be produced to be used in an adaptable solarairframe, examples of which are shown in FIGS. 2 and 3. In this case theairframe 400 may be made from a single or multiple flexible materials,such as plastic films, fabrics, composite films, etc. The airframe 400may be able to continuously change its shape and conform to variousspecific profiles, for example such as 401, 402, and 403 shown in FIG.4. Changes in the shape of the airframe 400 may be accompanied forexample in the reduction of its thickness, so that by transitioning fromprofile 401 to profiles 402 and 403 the airframe 400 gradually becomesthinner. This capability is attractive for example for use in theadaptable solar airframe 300. The airframe 400 made from non-stretchablematerials may maintain its surface area substantially constant, evenwhen transitioning from one shape to another. Nevertheless, the volume(and thickness) of 401 is greater than that of 402, and the volume (andthickness) of 402 is greater than that of 403. Correspondingly, thelarger is the volume of the airframe 400, the greater is its aerodynamicdrag. For any given volume and thickness of the airframe 400, its shapeis modified to reduce the parasitic air drag. Thus, it is important tomaintain the minimum possible volume at all time to conserve propulsionpower and minimize it to absolute minimum at night time or any othertime when the solar power system is not in use.

Alternatively, an expandable airframe 500 shown in FIG. 5 may beproduced that may be able to change not only the size of its volume, butalso the size of its surface, particularly the size of its externalsurface, i.e., the wetted area of the airframe. The airframe 500 that ismade from stretchable materials may take different shapes, such as forexample 501, 502, and 503 shown in FIG. 5. The operation of the airframe500 in this case may be similar for example to that of an expandableballoon, in which case the overall system may also include an air pumpfor providing additional internal air pressure. For example, FIG. 5shows an optional air pump/compressor 510 located inside the expandableairframe 500; alternatively, a pump may be located externally withrespect to the expandable airframe and attached to other parts of anairplane. One of the advantages of the expandable airframe 500 is thatthe volume of the airframe and therefore its aerodynamic drag may besignificantly reduced during nighttime when solar cells are not in use.The expandable airframe 500 may be advantageous, for example, in theadaptable solar airframe 200.

In addition, an airframe may be expandable not only in the vertical(along the Z axis) and lengthwise directions (along the flight directionor the X axis shown in FIG. 1), but also in the span-wise direction,i.e., along the axis perpendicular to both of the above directions (theY axis). The expandable airframe may be retractable, telescoping,collapsible, stretchable, flexible, foldable, and so on. For example,FIG. 6 shows an airframe 600 comprised of three telescoping sections610, 620 and 630 that may be folded or collapsed into a more compactform 601 shown in FIG. 7. The approach illustrated in FIGS. 4-6 aims toreduce aerodynamic drag during flight under any conditions, butparticularly under low-light or nighttime conditions by reducing thesize of solar collectors and an airframe containing them. The reductionin size can be accomplished by interleaving or folding segmented rigidelements, or by deflating, rolling, or folding flexible elements in boththe collector and airframe. FIG. 19 shows schematically an expandableairframe 1900 comprising an external shell or a canopy 1910 held inplace by expandable struts 1920. The flexible canopy 1910 may bereshaped by changing the length of the struts 1920. Alternatively, FIG.20 shows schematically an expandable airframe 2000 comprising anexternal shell or a canopy 2010 held in place by rigid rotating struts2020. In this case the shape of the airframe may be changed by turningthe struts 2020 and varying their relative angles. Of course, additionalstruts, structural elements and other approaches may be used inexpandable airframes.

In accordance with this invention, the solar power system 220 in theadaptable solar airframe 200 may include an exemplary solar power system800 shown in FIG. 8. The solar power system 800 comprises a carrier 810to which solar cells 820 (single or multi-junction) are attached, whichmay be either flat (i.e., planar) or nonplanar. The solar cells 820 arebroad area PV cells able to absorb both direct and scattered sunlight830 illuminating them and subsequently convert sunlight energy intoelectricity. The solar power system 800 may be used in combination witha tracking system, either single or dual axis, in order to increase itscollection efficiency and power production. Furthermore, in case thecarrier 810 and solar cells 820 are made of flexible thin-filmmaterials, the solar power system 800 may be used in place of the solarpower system 320 in the adaptable solar airframe 300.

The solar power system 220 may also include an exemplary solar powersystem 900 shown in FIG. 9. The solar power system 900 comprises anefficient solar PV cell or multiple cells 910 and an opticalconcentrator 920. The solar PV cell 910 may be an efficientmulti-junction solar cell, whereas the optical concentrator 920 may be alens, a Fresnel lens, a parabolic mirror, or combinations thereof. Insome cases it may be preferable to combine the cells 910 and theconcentrator in the same housing, so that the solar light 930 may betransferred from the concentrator to the cells through free space.Alternatively, the optical concentrator 920 may be a more complexapparatus, which combines light focusing elements with light guidingelements. In this case the cells 910 and concentrator 920 may be inseparate housings, so that the solar light 930 may be collected with theconcentrator in one location and then transmitted via light guidingelements, e.g., optical fiber or mirrors, to the cells in anotherlocation in the adaptable solar airframe. The solar power system 900primarily collects direct sunlight; therefore it may be mounted onto asun-tracking system, typically a dual-axis tracker.

In another example, a solar power system 1000 shown in FIG. 10 may beused. It comprises a frame 1010, plurality of solar cells 1020 andoptical concentrators 1030. As in solar power system 900, this systemrelies primarily on the direct sunlight and thus needs a sun tracker. Inthis case each concentrator 1030 focuses sunlight 1040 on itscorresponding solar cell 1020. Subsequently, the volume of the solarpower system 1000 may be reduced with respect to that of the solar powersystem 900 without changing its effective active area.

The solar power system 220 may also include an exemplary solar powersystem 1400 shown in FIG. 14. The solar power system 1400 comprises anarray of multiple (in this instance three) PV cells 1411, 1412, and1413, an optical concentrator 1420 and an optical dispersive element1425. The solar cells 1411-1412 may be single or multi-junction solarcells that are optimized to convert specific spectral parts of solarlight 1430 with high efficiency, for example by varying the opticalbandgap of the absorber materials in corresponding cells. Thus, the cell1411 may have the highest bandgap absorber and absorb theshort-wavelength part of solar spectrum. The cell 1412 may have themiddle bandgap absorber and absorb the middle portion of the solarspectrum and correspondingly the cell 1413 may have the lowest bandgapabsorber and operate in the longest wavelength portion of the sunlightspectrum. The optical concentrator 1420 may be a lens, a Fresnel lens, aparabolic mirror, or combination thereof. The dispersive element 1425may be one or more of a transmissive grating, a reflective grating, aholographic gating, a prism, or other type of dispersive elements. Thedispersive element 1425 serves the purpose of splitting the solar beaminto spatially separate parts of solar spectrum (slices), so that beam1431 may be the shortest wavelength portion, beam 1432 may be the middlewavelength portion and beam 1433 may be the longest wavelength portion.Each of these beams is focused onto the corresponding cell in the array.The solar spectrum may be split into any other number of spectral slices(other than three), for example two, four, five, etc. Accordingly, thenumber of solar cells in the array may be changed to match or correspondto the number of spectral slices. The solar power system 1400 may bemounted onto a sun-tracking system, preferably a dual-axis tracker.

FIG. 11 shows an exemplary adaptable solar airframe 1100, whichcomprises an expandable airframe 1110, a solar power system 1120 and asingle axis solar tracking system 1130. The expandable airframe 1110acts as a canopy and may be constructed from a transparent flexibleplastic held in place by flexible and retractable ribs and struts. Thesolar power system 1120 may be for example at least one flat PV module,e.g., like the solar power system 800 shown in FIG. 8, that can betilted towards the sun to provide optimum exposure. Alternatively, thesolar power system 1120 may include a concentrated solar PV system,similar for example to the solar power system 1000 shown in FIG. 10. Thesolar power system 1120 includes a single axis tracker, which allowstilting solar modules, panels and other elements of this solar powersystem in a single plane. In order to further increase solar exposure,the whole airframe 1110 may be tilted or rotated in another plane thatis not coplanar with the tilting plane of the tracking system 1130.Furthermore, an additional tracker may be added as shown in FIG. 12.FIG. 12 shows an exemplary adaptable solar airframe 1200, whichcomprises an expandable airframe 1210, a solar power system 1220 and adual axis solar tracking system comprising two orthogonal rotating axes1230 and 1240. The solar power system 1220 may include a plurality of PVmodules. These modules may be tilted in any direction using the internaldual axis tracking system to maximize solar exposure and thereforecollected solar energy, without rotating or moving the airframe 1200. Ofcourse, other implementations of this aspect of the invention arepossible. For example, FIG. 13 shows an adaptable solar airframe 1300that also comprises an adaptable airframe 1310 and a solar PV system1320 with a dual axis tracking system 1330. The solar PV system 1320 andthe dual axis tracking system 1330 may be as described above. In thiscase the transparent adaptable airframe 1310 may be for example a domeor a pod providing aerodynamic cover for an internal solar power system.

To illustrate advantages of the present invention, FIG. 15 and FIG. 16show planes with adaptive airframes having their positions adjusted withrespect to the relative position of the sun. FIG. 15 shows the head-onview of a plane 1500 with an adaptive solar airframe 1510 having the sunon its right wing at a zenith of a degrees. In order to reduce thecosine loss and maximize solar collection in this position, the adaptiveairframe 1510 may be rolled independently from the rest of the plane ata roll angle α matching the solar zenith, as shown in FIG. 15.Alternatively, the solar panels or other solar collectors inside theadaptive airframe 1510 may be tilted at angle α without changing theposition of the canopy or the outer shell of the airframe. FIG. 16 showsthe side view of a plane 1600 with an adaptive solar airframe 1610 withan internal solar power system 1615 flying directly into the sun at azenith of β degrees. In order to reduce the cosine loss and maximizesolar collection in this position, the solar power system 1615 may betilted independently from the rest of the airframe 1610 at a pitch angleβ matching the solar zenith, as shown in FIG. 16. The shape of theairframe 1610, particularly its external canopy, is optimized to containthe solar power system 1615 pitched at any specific angle and minimizeaerodynamic drag by having smooth streamlined body surface. Similarbenefits may be realized with adaptive airframes that have externalthin-film solar power systems mounted on their upper surfaces. Anadaptable solar airframe may in general compensate for any relativeangular position of the sun with respect to a plane by rotating itssolar collectors by appropriate roll and pitch angles and thus achievingmaximum collection efficiency. Current state-of-the-art approaches donot have these capabilities. Thus, at least some embodiments of thepresent invention improve solar collection efficiency by compensatingboth for the roll angle and for the pitch angle of the sun from idealoverhead (normal) incidence. The roll angle compensation is accomplishedby banking the solar collectors and airframes around the X axis. Thepitch angle compensation is accomplished by tilting solar collectorsaround the Y axis and thereby increasing the cross-section of theadaptable airframes to provide a greater area of capture of sunlightalong the flight direction. At each pitch angle, airframe's shape adaptsto provide nearly minimal drag for the given capture area, underexisting flight conditions.

To further illustrate the capabilities of an adaptive solar airframe,FIGS. 17A, 17B, and 17C, collectively referred to as FIG. 17, show anexemplary adaptive airframe 1700 that comprises an expandable airframe1710 and a solar power system 1720. FIGS. 17A, 17B, and 17C show how theexpandable airframe 1710 changes its shape to compensate for differentpitch angle of the sun. FIG. 17A shows the airframe 1700 adopting astreamlined but highest aerodynamic cross-section configuration tocollect light efficiently when the sun 1730 is incident along the flightaxis (directly in front or behind the aircraft). FIG. 17B shows asmaller cross-section configuration to collect light efficiently whenthe sun 1730 is at modest pitch angles (forward or rear) off the flightaxis. FIG. 17C shows a flat airframe configuration to collect lightefficiently when the sun 1730 is nearly perpendicular to the flightaxis. In these diagrams, solar collectors or panels of the solar powersystem 1720 are shown interior to the airframe, steered to be normal tothe incident sunlight, or retracted at night. Alternatively, PV panelsmay be attached directly on external airframe surfaces. A thirdalternative is to provide rigid or semi-rigid segmented collectors onthe interior of transparent adaptive airframes, but to simplify control,increase capture area, and reduce weight by relaxing the requirementthat the collectors be steered precisely normal to incident sunlight.

The adaptive solar airframes described above may be used as parts of aconventional airplane, such as airplane 100 shown in FIG. 1. In thiscase its fuselage, main wing or parts of the tail section may bemodified as an adaptive airframe with an integrated solar power system.However, it may be preferred to have an adaptive airframe having zerolift in order to minimize its effects on the flight characteristics of aplane. For example, parts of a fuselage may be used to produce zero-liftadaptive airframes. Alternatively, unconventional airplane designs maybe used to have additional zero-lift airframe sections, such as forexample airframes 1510 and 1610 in FIGS. 15 and 16, respectively. Thesesections may be rigidly attached to the fuselage, wings, or parts of thetail. The zero-lift airframe sections may have symmetric airfoilprofiles and zero attack angles (an attack angle is the angle between anairfoil and the flight direction), as shown for example in FIGS. 4, 5,and 16. Alternatively, an airplane may have storage compartments fromwhich the adaptable solar airframes may be extracted and expanded forsolar collection during daytime and then retracted and stored duringnighttime. In one example shown in FIG. 18, an adaptable solar airframe1810 may be made in the form of an inflatable, flexible pod covered withPV solar cells, so that it can be stored deflated inside the fuselage ortail at night and towed behind a plane 1800 fully inflated during theday.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof.

The invention claimed is:
 1. An adaptable solar airframe comprising: anexpandable body configured to expand during flight while maintaining anaerodynamic cross-section profile of an aerofoil that includes a largercross-sectional width over a center portion of the aerofoil than atleading and trailing portions of the aerofoil; and a flexible solarphotovoltaic (PV) system attached to a surface of the expandable body,wherein changes in shape of the expandable body are based on variableincident angle sunlight conditions during flight to optimize a capturearea of the flexible solar PV system during flight.
 2. The adaptablesolar airframe of claim 1, wherein the flexible solar PV systemcomprises broad area PV cells.
 3. The adaptable solar airframe of claim1, wherein the flexible solar PV system comprises a carrier made offlexible thin-film materials and solar cells made of flexible thin-filmmaterials attached to the carrier.
 4. The adaptable solar airframe ofclaim 1, wherein the flexible solar PV system comprises multi-junctionthin-film PV cells.
 5. The adaptable solar airframe of claim 1, whereinthe flexible solar PV system comprises planar solar cells.
 6. Theadaptable solar airframe of claim 1, wherein the expandable body issymmetric in shape and produces zero aerodynamic lift.
 7. The adaptablesolar airframe of claim 1, wherein a volume of the expandable body isvariable with the change in shape, and the shape is modified to minimizeparasitic drag for a given volume.
 8. The adaptable solar airframe ofclaim 1, wherein a surface area of the expandable body is able to remainconstant with the changes in the shape of the expandable body.
 9. Anaircraft comprising the adaptable solar airframe of claim
 1. 10. Theaircraft of claim 9, wherein the adaptable solar airframe isretractable.
 11. The aircraft of claim 9, comprising a storagecompartment in which the adaptable solar airframe may be stored, andfrom which the adaptable solar airframe may be extracted and expandedfor solar collecting.
 12. The aircraft of claim 9, wherein the adaptablesolar airframe is an inflatable flexible pod.
 13. The aircraft of claim12, wherein the inflatable flexible pod is towed by the aircraft. 14.The aircraft of claim 9, wherein the adaptable solar airframe comprisesa part of one or more of a fuselage, a wing, or a tail section.
 15. Anadaptable solar airframe comprising: an expandable body configured toexpand during flight while maintaining an aerodynamic cross-sectionprofile of an aerofoil that includes a larger cross-sectional width overa center portion of the aerofoil than at leading and trailing portionsof the aerofoil; and an integrated solar power system comprisingthin-film photovoltaic (PV) solar cells and modules integrated with asurface of the expandable body, wherein changes in shape of theexpandable body are based on variable incident angle sunlight conditionsduring flight to optimize a capture area of the integrated solar powersystem during flight.
 16. The adaptable solar airframe of claim 15,wherein the thin-film PV solar cells and modules are imbedded into askin of the airframe.
 17. An airplane comprising the adaptable solarairframe of claim
 15. 18. The airplane of claim 17, wherein theadaptable solar airframe has zero aerodynamic lift.
 19. The airplane ofclaim 17, wherein the adaptable solar airframe is a pod that can betowed behind the airplane.
 20. The airplane of claim 19, furthercomprising a storage compartment into which the adaptable solar airframemay be retracted and stored or from which it may be extracted andexpanded.